Isotope Shifts in the 7s$\to$8s Transition of Francium: Measurements and Comparison to $\mathit{\text{ab}}\mathit{\text{initio}}$ Theory

We observe the electric-dipole forbidden $7s\to 8s$ transition in the francium isotopes ${}^{208-211}$Fr and ${}^{213}$Fr using a two-photon excitation scheme. We collect the atoms online from an accelerator and confine them in a magneto optical trap for the measurements. In combination with previous measurements of the $7s\to 7p_{1\mathrm{/}2}$ transition we perform a King Plot analysis. We compare the thus determined ratio of the field shift constants (1.230 $\pm$ 0.019) to results obtained from new ab initio calculations (1.234 $\pm$ 0.010) and find excellent agreement.